1. Field of the Invention
The present invention relates to a safety valve unit that is generally called “pressure relief valves” in the art, and more particularly to a pressure relief valve unit that is inline-connected with a fluid circuit to control the circuit pressure in a manner such that, when a fluid passage in the circuit is subjected to a fluid pressure more than a predetermined set point called “cracking pressure”, the pressure relief valve of the unit is opened to relieve the amount of fluid necessary to maintain such a predetermined set point pressure in the circuit, wherein: the pressure relief valve or safety valve unit also functions as a check valve to prevent fluid from flowing in the reverse direction in the circuit; and, the subject safety valve unit is capable of being adjusted in cracking pressure from the outside.
2. Description of the Related Art
In general, a safety valve unit of a conventional type comprises: a stationary valve body; a spring-biased movable valve element; and, a valve spring for biasing the valve element, wherein both the valve element and the valve spring are housed in the valve body. Cracking pressures for valve openings or closures of the safety valve, are determined by a biasing force exerted by the valve spring. Consequently, a simple adjustment in biasing force of the valve spring is enough to adjust the set point of cracking pressure in the safety valve unit.
Examples of the conventional type of safety valve unit, which are adjustable in cracking pressure, are disclosed in Japanese Patent application Laid-open Nos.: 2003-329155; and 2003-322267, for example. One of these examples of the conventional safety valve unit is shown in FIG. 6. In this example, a valve spring support 54 for supporting a distal end portion of a valve spring 50 located remote from a valve element 52 is displaced relative to a valve body 51 so that a resilient force or biasing force exerted by the valve spring 50 is adjusted.
In this example of the prior art, as is clear from FIG. 6, however, it is necessary for a user to dismount or disassemble the safety valve unit from the fluid line in order to realize a displacement of the valve spring support 54 relative to the valve body 51, because the valve spring support 54 is disposed inside the valve body 51 and threadably engaged with the same body 51. Further, after completion of such disassembly of the safety valve unit, it is necessary for the user to assemble the thus disassembled safety valve unit into the fluid line again. Such disassembling and assembling operations of the safety valve unit are very hard and cumbersome.
In addition, in the conventional safety valve unit having the above construction: the valve body 51 has its opposite end portions threadably engaged with a pair of housings 55, 56 each provided with a coupler in its axially outer end portion; and a sealing member 57 is interposed between an outer peripheral surface of the valve body 51 and an inner peripheral surface of each of the housings 55, 56 to establish a liquid-tight or hermetical compression seal therebetween. However, such sealing member 57 tends to become worse in quality with elapsed time due to wear and like adverse factors, which leads to fluid leakage in the prior art safety valve unit.